werkzeug.contrib.securecookie bug with Python 3

[simonw]

In [1]: from werkzeug.contrib.securecookie import SecureCookie
In [2]: SecureCookie({'hello': 'world'}, 'cookie-secret').serialize()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-3fd8875133b9> in <module>()
----> 1 SecureCookie({'hello': 'world'}, 'cookie-secret').serialize()

.venv/lib/python3.6/site-packages/werkzeug/contrib/securecookie.py in __init__(self, data, secret_key, new)
    153         # no longer performs an implicit string conversion on hmac
    154         if secret_key is not None:
--> 155             secret_key = bytes(secret_key)
    156         self.secret_key = secret_key
    157         self.new = new

TypeError: string argument without an encoding

It looks like this code assumes Python 2:

werkzeug/werkzeug/contrib/securecookie.py

Lines 150 to 155 in 79b33a7

	def __init__(self, data=None, secret_key=None, new=True):
	ModificationTrackingDict.__init__(self, data or ())
	# explicitly convert it into a bytestring because python 2.6
	# no longer performs an implicit string conversion on hmac
	if secret_key is not None:
	secret_key = bytes(secret_key)